Thermal oxidizers, and in particular, regenerative thermal oxidizers, have been used downstream of drying systems to remove Volatile Organic Compounds (VOC's) and carbon monoxide (CO) emissions. These drying systems may be drying any of a myriad of materials, such as green wood, wood fiber, coffee beans, agricultural products and other materials to lower the moisture content of the raw material so it can be turned into a final product. Furthermore, these drying systems often use a biomass fuel furnace to provide heat to the dryer. The abatement system often utilized to meet stringent air quality standards typically includes a device or devices to remove particulate and a device to remove gaseous organic compounds. For example, particulate removal can be accomplished via cyclones, baghouses, scrubbers and more typically dry or wet electrostatic precipitators. These particulate devices may operate by themselves, but when VOC, CO and other gaseous organic compounds must be removed, they are utilized as a pre-filter for an oxidizer. Additional manufacturing steps such as in the pressing process also release VOC's that can be treated in additional oxidizers. The most common type of oxidizer is a regenerative thermal oxidizer (known as an RTO) which can have up to about 98 percent energy recovery of the oxidized gas. Oxidizers have been employed to abate volatile organic compounds (VOC's) from industrial processes and this practice is well known.
In some industries, the amount of VOC's exhausted contain a high enough caloric value to equal the thermal energy requirement of the oxidizer, so once operating temperature is achieved by the oxidizer burner, the oxidizer burner turns off or goes to low fire with the balance of the energy necessary for combustion coming from the combustion of the VOC's in the process gas. This is typically not the case in other industries such as panel board manufacturing. As a result, since the mid 1990's, companies have searched for alternate ways to operate the oxidizer other than by consuming fossil fuels (such as natural gas or propane). One such proposal is to build a biomass gasifier and fire the oxidizer burner with the “producer” or “syngas”, as it is referred to, in the RTO burner (hereinafter “syngas”). This idea has been impractical for several reasons. The typical syngas has a much lower heating value then natural gas, typically one tenth thereof, and therefore does not operate in a conventional burner very well. The heating value of the gas also varies over time. For this reason, the large volume of syngas, if injected directly into the burner chamber, would affect the mass balance of the oxidizer, resulting in a drop of thermal efficiency. The most important reason that this idea has been impractical is that a stand-alone biomass gasifier fitted with the required equipment to condition the syngas prior to delivery to the oxidizer is very expensive and has a very long return on investment.
Continuous efforts have been undertaken by the present inventors to devise a more practical and economical way to supplement or replace the fossil fuel used in an oxidizer. It was observed that many of these types of manufacturing facilities, when constructed new in recent years, were installing biomass furnaces to heat the dryers and hot oil systems. These furnaces were replacing older technology such as suspension burners that require the use of dry and fine small particles of wood to create a fire for heating. Instead, these furnaces can combust wet (typically 25% to 50% moisture) scrap material such as bark, pine needles and hogged stumps. Many older facilities have retrofitted this type of furnace to lower the cost of the biomass (scrap) that is used to heat the process. It was further observed that these furnaces typically have a two step combustion process. Step one involves a pile or mound of the material burning at the bottom of the furnace with minimal combustion air added from the side or below the pile. This slow combustion and minimal air causes a low temperature and a reduced (low oxygen) environment around the fuel. The gas coming off has similar properties including caloric value to those produced in gasifier systems. This gas then travels a distance within the furnace until secondary air is added in step 2, completing combustion and producing high grade heat for the drying process.
It would be desirable to provide a process of effectively and efficiently using the syngas produced in a gasifier to operate a downstream RTO, such as by maintaining the RTO at operating temperature while minimizing or eliminating the use of fossil fuel.